Various communication systems are used in the world presently. Such communication systems are not necessarily the same because old and new communication systems are mixed, and the frequency and detailed specifications differ from one country to another. Therefore, to use a portable terminal in the world, the terminal needs to support various types of systems, and it is required to have therein two or more high-frequency power amplification modules supporting such systems. However, this leads to the increase in size and weight of the portable terminal. If a high-frequency amplifier (multimode amplifier), which supports two or more communication systems and has an efficiency increased, is materialized as means to solve the problem, it will be possible to reduce the size and weight of a portable terminal.
To realize the above, various methods have been reported. The examples of such methods are described in Patent Documents 1 to 4, which are to be noted later. Of those documents, Patent Documents 1 to 3 each describe a technique which includes using a Doherty amplifier. In regard to such technique, a high efficiency can be expected in a range of middle to high outputs, which can be achieved even when the output level is changed. On the other hand, Patent Document 4 describes a technique to reduce the fluctuation in output power incident to the variation in load.
In general, such RF power amplifier requires a high power-added efficiency and a high output power. Nonpatent Document 1, which is to be noted later, contains the description on an RF power amplifier in accordance with an architecture which is referred to as Wilkinson type architecture. This is because small amplification devices can provides a high gain, low matching Q factor (broad band), good phase linearity, and cost saving, even if a large amplification device is available. In accordance with the architecture, an input coupler is arranged for input terminals of such small power amplifiers, and an input power is divided to the input terminals. In addition, an output coupler is arranged for output terminals of the small power amplifiers, output powers is coupled into one output power. The input and output couplers each incorporate a λ/4-wavelength line which creates a 90-degree phase shift. As the hybrid coupler separates two power amplifiers from each other, even when one amplifier breaks down, the other can work. In Nonpatent Document 1, which is to be noted later, it is described that this RF power amplifier further provides a fixed input impedance, and allows the cancellation of odd harmonics and the cancellation of an inter-modulation distortion of an opposite direction.
In addition, Nonpatent Document 2 contains the description on an RF power amplifier in accordance with DD-CIMA (Divided Device and Collectively impedance-matched amplifier) architecture, which incorporates an LC resonant circuit instead of the λ/4-wavelength line as described in the Nonpatent Document 1. Also in this architecture, small amplification devices are used instead of one large amplification device.
Further, Nonpatent Document 1 contains the description on an RF power amplifier in accordance with an architecture which is referred to as Doherty type architecture; the RF power amplifier includes a combination of a main power amplifier biased in Class B and an auxiliary power amplifier biased in Class C. In accordance with the architecture, only the main power amplifier works, and the auxiliary power amplifier remains cut off when an input power is low. When an input amplitude rises to a middle input power, the auxiliary power amplifier is activated. A λ/4-wavelength impedance converter is connected between outputs of the two power amplifiers, which achieves a high power-added efficiency by means of load modulation by a large load when the input power is low and a small load when the input power is high.
Also, Nonpatent Document 3 contains the description that in a Doherty type RF power amplifier, two identical devices are used as a main power amplifier biased in Class AB and as an auxiliary power amplifier biased in Class C.
Further in Patent Document 4 is introduced an RF-MEMS switch based on MEMS (Micro Electromechanical System). It is reported that the switch exhibits excellent high-frequency characteristics and can be designed and manufactured by making use of a technology similar to VLSI technology.
Patent Documents 1-4 and Nonpatent Documents 1-4 are as follows.
Patent Document 1: U.S. Pat. No. 6,374,092.
Patent Document 2: JP-A-2004-173231.
Patent Document 3: U.S. Pat. No. 6,204,731.
Patent Document 4: U.S. Pat. No. 6,954,623.
Nonpatent Document 1: Frederic H. Raab et al, “Power Amplifier and Transmitter for RF and Microwave”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 3, MARCH 2002, PP. 814-826.
Nonpatent Document 2: Isao Yoshida et al, “A 3.6V 4 W 0.2 cc Si Power-MOS-Amplifier Module for GSM Handset Phones”, 1998 IEEE International Solid State Circuits Conference DIGEST OF TECHNICAL PAPERS, PP. 50-51.
Nonpatent Document 3: Ingo Dettmann et al, “Comparison of a Single-Ended Class AB, a Balance and a Doherty Power Amplifier”, 2005 IEEE Proceedings Asia-Pacific Microwave Conference Proceedings, VOL. 2, 4-7 Dec. 2005, PP. 1-4.
Nonpatent Document 4: Elliot R. Brown, “RF-MEMS Switches for Reconfigurable Integrated Circuits”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 11, NOVEMBER 1998, PP. 1868-1880.